In general, a reluctance machine is an electric machine in which torque is produced by the tendency of its movable part to move into a position where the inductance of an excited winding is maximized. The general theory, design and operation of switched reluctance machines is well known and is discussed, for example, in the paper "The Characteristics, Design and Applications of Switched Reluctance Motors and Drives" by Stephenson and Blake and presented at the PCIM '93 Conference and Exhibition at Nurnberg, Germany, Jun. 21-24, 1993.
FIG. 1 illustrates a typical switched reluctance machine having a stator 10 including six projecting stator poles 11-16 that define a principal stator axis (extending outwardly from FIG. 1). A rotor 18 is coupled to a rotatable shaft co-axial with the principal axis of the stator. In FIG. 1, the rotor is positioned within the bore formed by the stator and the inwardly pointing stator poles 11-16 and is mounted on a shaft (not shown) that is mounted on bearings and is free to rotate. The rotor 18 has a number of outwardly extending projections 19 which form the rotor poles.
Associated with each stator pole is a wound coil of wire. In the illustrated machine, the two coils of opposing stator poles are coupled together to form three phases: phase A (coils from poles 11 and 14); phase B (coils from poles 12 and 15); and phase C (coils from poles 13 and 16). In the example illustrated in FIG. 1, when phase A is energised, current will flow through its coils such that stator pole 11 becomes, for example, an inward-pointing electromagnet of positive polarity and stator pole 14 becomes an inward-pointing electromagnet of negative polarity. These electromagnets will produce a force of attraction between the energised stator poles and the rotor poles which will produce a torque. By switching energisation from one phase to another, the desired torque may be maintained regardless of the angular position of the rotor. By switching the energisation of the phase windings to develop positive torque, the machine may be operated as a motor; by energisation of the phase windings to develop a retarding torque the machine may be operated as a brake or generator.
For the sake of illustration, a simple form of machine having six stator poles and two rotor poles (i.e. a 6/2 machine) is shown. Those skilled in the art will recognize that other combinations are well-known. The present invention applies equally to such machines. Moreover, the present invention is applicable to inverted machines, where the stator is positioned within the bore of an outer rotating rotor, and to linear machines, in which the movable member moves linearly with respect to the stator. In the art the movable member of a linear motor is also commonly referred to as a rotor.
When a switched reluctance machine is running, the torque (and other machine performance parameters) may be adjusted by monitoring the rotor's position, energising one or more phase windings when the rotor is at a first angular position, referred to as the "turn-on angle", and then de-energising the energised windings when the rotor rotates to a second angular position, referred to as the "turn-off angle". The angular distance between the turn-on angle and the turn-off angle is known as the "conduction angle", constituting the limits of an active period in which the phase winding is energised.
At standstill and at low speeds, the torque of a switched reluctance machine can be controlled by varying the current in the energised phases over the period defined by the turn-on and turn-off angles. Such current control can be achieved by chopping the current using a current reference with phase current feedback. Such current control is referred to as "chopping mode" current control. Alternatively, pulse width modulation (PWM) voltage control may be used. Chopping mode current control and PWM control strategies are generally understood and chopping mode current control is generally described below.
FIG. 2A illustrates an exemplary current in a phase winding when chopping mode current control is used when the switched reluctance machine is operating as a motor. As is illustrated in FIG. 2A, the phase is initially energised at a point corresponding to the turn-on angle and current begins to increase until it reaches the current reference. At that point, the current is chopped by a controller, de-energising the phase winding. The current drops until the phase winding is again re-energised and the process repeats. As indicated in FIG. 2A, in the chopping mode, the overall shape of the current waveform defines a substantially rectangular region where the beginning and end points of the rectangular region generally correspond to the turn-on and turn-off angles, defining between them the conduction angle.
As the angular speed of the motor increases, a point is reached where there is insufficient time for more than a single pulse of current to occur during each phase period. Accordingly, at these speeds pulse width modulation or chopping strategies are ineffective. The torque of the motor is then commonly controlled by controlling the position and duration of the voltage pulse applied to the winding during the phase period. Because the single pulse of voltage is applied during each phase period, this form of control is referred to as "single-pulse control". This is illustrated in FIG. 2B.
FIG. 3 generally illustrates power circuitry that may be used to control the energisation of a phase winding for both chopping mode and single-pulse mode current control. A phase winding 30 is coupled to a source of DC power provided through a DC bus, comprising positive and negative rails 31/32, by upper switching device 33 and lower switching device 34. Return diodes 35 and 36 provide a current path from the DC bus through the phase winding 30 when switching devices 33 and 34 are opened. As those skilled in the art will appreciate, phase winding 30 is generally energised by closing switches 33 and 34, thus coupling the phase winding to the DC bus.
The circuit illustrated in FIG. 3 may be used to implement chopping mode current control as follows: when the rotor reaches an angular position that corresponds to the turn-on angle, switches 33 and 34 are closed. The phase winding 30 is then coupled to the DC bus, causing an increasing magnetic flux to be established in the motor. It is the magnetic field associated with this flux which acts on the rotor poles to produce the motor torque. As the magnetic flux in the machine increases, current flows from the DC supply as provided by the DC bus, through the switches 33 and 34 and through the phase winding 30.
The current flowing through the phase winding 30 is sensed by a current sensor or other device (not shown) that provides a signal corresponding to the magnitude of the phase current. The signal corresponding to the phase current is then compared with a signal representing a reference current. When the actual current in the phase winding exceeds the reference current, the phase winding is de-energised by opening one or both of switches 33 and 34. When both switches 33 and 34 are opened, the current in the phase winding 30 transfers from switches 33 and 34 and flows through the diodes 35 and 36. The diodes 35 and 36 then apply the DC voltage appearing on the DC bus in the opposite sense, causing the magnetic flux in the machine (and therefore the phase current) to decrease. When the current decreases below the reference current by a predetermined value, the phase is re-energised and the current again begins to increase.
The process of energising the phase winding 30, de-energising it when the phase current exceeds the reference current, and re-energising it when the phase current drops below the reference current by a predetermined value, repeats itself during the interval defined by the turn-on and turn-off angles. Typically, when the rotor reaches an angular position corresponding to the turn-off angle, switches 33 and 34 are opened, and the phase current is allowed to drop to zero. At that point the diodes 35 and 36 turn off, disconnecting the phase winding from the power supply.
As those skilled in the art will appreciate, the above discussion of current control is but one example of a current control strategy that may be used and that alternative strategies, e.g., strategies including freewheeling, may also be used. The circuit illustrated in FIG. 3 may be also used to implement single-pulse mode current control.
The inherently inductive nature of a phase winding can lead to problems with transient voltage in the forms of spikes when switching a voltage across the winding. These spikes have a much larger peak magnitude than the switched voltage and a very high rate of increase and decrease. The magnitude of the voltage can damage the switch element. To counter this it is known to use a so-called `snubber` circuit connected across the switch to suppress the transient voltage spikes in the switch. In known snubber circuits the rate of increase and decrease in the voltage transient is typically suppressed without effecting appreciably the responsiveness of the switch to apply the new voltage level to the phase winding at the desired time.
As the above discussion indicates, as a switched reluctance motor (or generator) operates, magnetic flux is continuously increasing and decreasing in different parts of the machine. This changing flux will occur in both chopping mode and single-pulse current control. The changing flux results in fluctuating magnetic forces being applied to the ferromagnetic parts of the machine. These forces can produce unwanted vibration and noise. One major mechanism by which these forces can create noise is the ovalising of the stator caused by forces across the airgap. Generally, as the magnetic flux increases along a given diameter of the stator, the stator is pulled into an oval shape by the magnetic forces. As the magnetic flux decreases, the stator springs back to its undistorted shape. This ovalising and springing back of the stator can cause unwanted vibration and audible noise.
In addition to the distortions of the stator by the ovalising magnetic forces, unwanted vibration and acoustic noise may also be produced by abrupt changes in the magnetic forces in the motor. The abrupt application or removal of magnetic force can cause the stator to vibrate at one or more of its natural resonance frequencies. In general, the lowest (or fundamental) natural frequency dominates the vibration, although higher harmonics can be emphasized by repeated excitation at the appropriate frequency.
In addition to the stator distortions resulting from the ovalising and vibration phenomena described above, the fluctuating magnetic forces in the motor can distort the stator in other ways, as well as distorting the rotor and other ferromagnetic parts of the machine. These additional distortions are another potential source of unwanted vibration and noise.
Although the problem of unwanted acoustic noise and vibration has been recognized, known control systems for reluctance motors do not adequately solve the problem. For example, the general problem of acoustic noise in switched reluctance motor systems is discussed by C. Y. Wu and C. Pollock in "Analysis and Reduction of Vibration and Acoustic Noise in the Switched Reluctance Drive", Proceedings of the Industry Applications Society, IAS '93 Conference, Toronto, Oct. 2-8 1993, pp. 106-113.
In general, the method suggested by Wu and Pollock involves control of the current in the phase winding such that the current is controlled in two successive switching steps with the second switching step occurring approximately one-half of a resonance cycle after the first where the resonance cycle is defined by the natural frequency of the machine. This approach is typically implemented by switching off one of the power devices at a first point in time to cause a first stepped reduction in applied voltage, and then later switching off the second power device. Between the time when the first switching device is switched off and the second switching device is switched off, the current is allowed to freewheel through a freewheeling diode and the second switching device.
The two-step voltage-reduction approach to noise reduction in switched reluctance motors discussed above suffers from several limitations and disadvantages. The two-step voltage-reduction approach limits the flexibility to dynamically adjust the freewheeling period for each phase cycle. As discussed above, in the two-step voltage-reduction approach, the duration of the freewheeling period is selected to reduce the noise produced by the system. There are many instances when it would be desirable to optimize the freewheeling duration according to other criteria.
An additional limitation of the two-step voltage-reduction approach, and other approaches that utilize freewheeling to reduce noise, is that, since there is typically only one freewheeling period per phase energisation cycle, freewheeling generally reduces noise produced by only a single frequency of the motor system. Freewheeling to reduce noise at one frequency does not necessarily reduce noise produced by other frequencies in motor systems that have more than one resonance frequency. Accordingly, such approaches do not reduce many of the frequencies at which unwanted noise is produced. A further disadvantage with the freewheeling approach is that there are several motor switching circuits that simply do not allow freewheeling. These systems cannot use freewheeling to reduce noise.
U.S. Pat. No. 5,461,295 (Horst) discloses apparatus for controlling the current profile in a switched reluctance motor. The phase winding current is initially at a first level in a first part of the active period of the switch cycle. Thereafter, it is linearly reduced over a second part of the active period. When the current is to decay due to switching off the switches at the end of the active period, the transition between the linearly reducing current in the second part of the active period and the ramp of the decay of the current is less abrupt, producing a smoother transition.